The disclosed subject matter relates generally to semiconductor device manufacturing and, more particularly, to a method and apparatus for performing double exposure photolithography using a single reticle.
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises four operations: layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced; patterning, or removing selected portions of added layers; doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer. Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process.
The fabrication process generally involves processing a number of wafers through a series of fabrication tools. Each fabrication tool performs one or more of the four basic operations. The four basic operations are performed in accordance with an overall process to finally produce wafers from which the semiconductor devices are obtained.
Of these four operations, patterning is considered to be an important step affecting the performance of the completed devices. Patterning is known to those in the art by many names. Other names for patterning include photolithography, photomasking, masking, oxide removal, metal removal, and microlithography. The term “photolithography” will hereafter be used to refer to patterning operations. Photolithography typically involves a machine called an “exposure tool,” or sometimes also called a “stepper” or a “scanner”. An exposure tool positions a portion of a wafer being processed under a “photomask.” The photomask is usually a reticle, which is a copy of a pattern created in a layer of chrome on a glass plate. Light is then transmitted through the reticle onto a thin layer of material called photoresist previously added to the wafer. The chrome blocks the light while the glass allows it to pass. This type of reticle is referred to as a binary mask. Another type of photomask is a “phase shift” photomask that shifts the phase of the light waves so that the projected image of the photomask has an improvement of one or more image characteristics (e.g., contrast, edge definition, etc.) as compared with the same pattern from a binary photomask. In some cases, multiple reticles are used for a single patterning step. The use of multiple reticles allows patterns to be formed that have pitches smaller than would be achievable with a single reticle.
It is common in photolithography for the exposure field size to be larger than the stepping distance on the wafer. To address this issue, fabricators typically generate a flare map for the reticle that estimates the intensity components due to the overexposure. However, conventional flare maps only consider the overexposure due to the patterning of the subject die location. In an actual fabrication process, the overexposure can also be affected by the patterning of neighboring die.
This multiple exposure approach leads to overlapping frame patterns around the border of the field. In situations where the reflection of “dark” areas on the mask cannot be ignored, such as extreme ultraviolet (EUV) lithography, this over exposure will cause the resist features in this area to be printed too small.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.